The present invention relates to an advantageous process for the hydrogenation of phenols to give the corresponding cyclohexanones using palladium-on-carbon catalysts.
Processes for the hydrogenation of phenols over palladium-on-carbon catalysts to give cyclohexanones are known in principle (see, for example, DE-A 2 909 780, DE-A 2 530 759 (G.B. 1,512,497) and U.S. Pat. No. 2,829,166).
In general, to achieve a good selectivity, a hydrogen pressure of less than 20 bar and a reaction temperature in the range from 120 to 250.degree. C. are sought and an alkaline compound is added to the hydrogenation mixture. The catalyst can, if desired, be treated with alkali metal ions such as Na or K ions (see, for example, German Auslegeschrift 1 144 267 (U.S. Pat. No. 3,076,810)). The use of phenoxides and bicarbonates is described in FR-A 2 372 136 (G.B. 1,563,044).
According to DE-A 2 909 780, sodium carbonate, borax and/or sodium acetate are used for the hydrogenation of p-tert-amylphenol. At the same time, it is indicated here that the use of palladium catalysts having metal surface areas of 15 m.sup.2 /g and more is promising. Preference is given to using palladium-on-carbon catalysts containing 5% by weight of palladium, based on a support which has a surface area of 800 m.sup.2 /g.
A disadvantage of these known processes is that only solvents which are difficult to handle from a technical and/or toxicological point of view are known for the reaction of phenols having high melting points (see, for example, EP-A 731 075). It would be desirable to have solvents which are nontoxic and safe to handle and can be separated off physically without problems and at little expense.
In addition, long reaction times are frequently required, particularly when phenols which are liquid at the hydrogenation temperature and no solvents are used.